1. Field of Invention
The present invention generally relates to photovoltaic cells. More particularly, the invention relates to the formation and protection of metallization layers on such photovoltaic cells.
2. Summary of the Prior Art
Silicon photovoltaic cells essentially comprise a semiconductor substrate of one conductivity type having a shallow p-n junction formed adjacent one surface thereof.
One method of making photovoltaic solar cells involves provision of semiconductor substrates in the form of flat sheets or wafers having a shallow p-n junction adjacent one surface thereof (commonly called the "front surface"). Such substrates, which may include an insulating anti-reflection ("AR") coating on their front surfaces, are commonly referred to as "solar cell blanks". The anti-reflection coating is transparent to solar radiation. In the case of silicon solar cells, the AR coating is often made of silicon nitride or an oxide of silicon or titanium. Preferably, but not necessarily, the silicon nitride is formed by a plasma deposition process.
A typical solar cell blank may take the form of a rectangular EFG-grown silicon substrate of p-type conductivity having a thickness in the range of 0.012 to 0.016 inches and a p-n junction located about 0.5 microns from its front surface, and also having a silicon nitride coating about 800 Angstroms thick covering its front surface. Equivalent solar cell blanks also are well known, e.g. those comprising single crystal silicon substrates and cast polycrystalline silicon substrates.
The cells require electrical contacts (sometimes referred to as "electrodes") on both the front and rear sides of the semiconductor substrate in order to be able to recover an electrical current from the cells when they are exposed to solar radiation. These contacts are typically made of aluminum, silver, or nickel. For example, a common arrangement with solar cells having a silicon substrate is to make the rear contact of aluminum and the front contact of silver.
The contact on the front surface of the cell is generally made in the form of a grid, comprising an array of narrow fingers and at least one elongate bus that intersects the fingers. The width, number, and arrangement of the fingers is such that the area of the front surface adapted for exposure to solar radiation is maximized. Further to improve the conversion efficiency of the cell, an AR coating as described above is applied at least to those areas of the first side of the cell that are not covered by the front contact.
Aluminum is preferred for the rear contact for cost and other reasons. The rear contact may cover the entire rear surface of the solar cell blank, but more commonly it is formed so as to terminate close to but short of the edges of the blank. However, the exposed surface of an aluminum contact tends to oxidize in air, making soldering difficult. Therefore, to facilitate soldering, it has been found useful additionally to provide apertures in the aluminum coating, with silver soldering pads being formed in those apertures so as to slightly overlap the adjacent aluminum layer. These silver pads form ohmic bonds with the underlying substrate and also low resistance electrical connections with the aluminum contact, and are used as sites for soldering the connecting copper ribbons to the rear contact. This latter arrangement is more efficient than soldering the copper ribbons directly to the aluminum contact. Such a contact arrangement is disclosed in PCT International Publication No. WO 92/02952, based on U.S. patent application Ser. No. 07/561,101, filed Sep. 1, 1990 by Frank Bottari et al. for "Method Of Applying Metallized Contacts To A Solar Cell" (U.S. Pat. No. 5,151,386, issued 29 Sep. 1992). The front and rear contacts may be formed in various ways, but preferably they are formed by a paste printing/firing technique which involves printing a selected metal-containing paste or ink onto each surface of the solar cell blank and then firing that paste or ink in a suitable predetermined atmosphere so as to cause the metal constituent of the paste or ink to bond to the blank and form an ohmic contact therewith. The paste or ink comprises an organic vehicle in which particles of the selected metal are dispersed, and the firing is conducted so that the vehicle's components are removed by evaporation and/or pyrolysis.
The printing may be conducted in various ways, e.g., by silk screen printing, ad printing or direct write printing techniques. One suitable pad printing technique is disclosed in PCT International Publication No. WO 92/02952, supra. U.S. patent application Ser. No. 666,334, filed 7 Mar. 1991 by Jack I. Hanoka and Scott E. Danielson for "Method And Apparatus For Forming Contacts" (U.S. Pat. No. 5,151,377, issued 29 Sep. 1992), discloses an improved method for direct writing a thick ink film onto the front surface of a solar cell blank. The teaching of those patents are incorporated herein by the foregoing reference thereto.
For purposes of clarification and definiteness, as used herein the terms "ink" and paste" are to be construed as essentially synonymous terms for describing fluid printing materials since they are used interchangeably by persons skilled in the art, although the term "ink" suggests a lower viscosity than the term "paste". In this connection, it is to be appreciated that the viscosity of the fluid material is adjusted according to the manner in which it is applied, e.g., silk screen printing, pad printing, and direct write printing. Also, the terms "metal paste" and "metal ink" are to be construed as denoting a metal-rich fluid comprising a selected metal in the form of discrete particles dispersed in an organic vehicle that is removable or destroyable on heating, e.g., via evaporation and/or pyrolysis. The metal paste may optionally contain a glass frit. The vehicle typically comprises an organic binder and a solvent of suitable properties, e.g., ethyl or methyl cellulose and Carbitol or terpineol. Thus, the term "aluminum metal paste" is to be construed as denoting a fluid aluminum-rich composition comprising aluminum particles dispersed in an organic vehicle. Further, the term "glass frit paste" denotes fluid compositions comprising a selected glass frit dispersed in an organic vehicle of the type previously described, and the term "metal/glass frit paste", e.g., a silver metal/glass frit paste, is to be construed as a metal paste that essentially comprise a selected glass frit in a predetermined amount on an eight percent basis.
Heretofore a preferred method of forming aluminum back contacts by the paste printing/firing technique has been to coat the rear side of the solar cell blank with an aluminum metal paste co-rising an aluminum powder dispersed in an organic vehicle so that the total weight of the aluminum powder in the paste applied to the blank is between about 0.8-2.0 mg/cm.sup.2 of coated substrate surface, and then firing that material (with or without a previous drying procedure) in a non-oxidizing atmosphere, e.g., nitrogen, under conditions adequate to evaporate and/or pyrolyze the organic vehicle and also cause the aluminum to alloy and fuse to the silicon substrate.
The alloying process involve melting the aluminum particles and the adjoining region of the substrate, and then cooling the solar cell blank to effect re-crystallization of the melted region of the substrate. The re-crystallized region comprises silicon highly doped with aluminum. The firing and cooling produces an aluminum contact on the rear surface of the substrate that is mechanically and electrically bonded to the re-crystallized region of the silicon substrate.
The grid-shaped contact on the front surface has been formed in various ways. For example, in some cases the grid contact is formed by a paste printing/firing method, and then covering at least those portions of the front surface of the substrate not covered by the grid contact with an AR coating.
Another approach comprises first coating the semiconductor substrate with an AR coating, and thereafter forming the grid contact. This latter approach has been practiced in two different forms. One way involves chemically etching away portions of the anti-reflection coating so as to expose areas of the front surface of the semiconductor substrate in the desired grid electrode pattern, and then forming the grid contact on the front surface in the region where the anti-reflection coating has been etched away.
The second way of forming the front contact utilizes the so-called "fired-through" method. That method utilizes a solar cell blank having an AR coating on its front surface and comprises the following steps: (1) applying a coating of a metal/glass frit paste to the surface of the AR coating in a predetermined pattern corresponding to the configuration of the desired grid electrode, and (2) heating the coated solar cell blank to a temperature and for a time sufficient to cause the metal/glass frit composition to dissolve and migrate through the anti-reflection coating and then form an ohmic contact with the underlying front surface of the substrate.
The "fired through" method of forming silver contacts is illustrated by PCT Patent Application Publication WO 89/12312, published 14 Dec. 1989, based on U.S. patent application Ser. No. 205,304, filed 10 Jun. 1988 by Jack Hanoka for an "Improved Method of Fabricating Contacts or Solar Cells". The concept of firing metal contact through an anti-reflection dielectric coating also is disclosed in U.S. Pat. No. 4,737,197, issued to Y. Nagahara et al. for "Solar Cell with Paste Contact".
In one prior art method of manufacturing solar cells having aluminum back contacts a so-called "double-fire" process is utilized. In that process, an aluminum metal ink is deposited on the rear surface of a solar cell blank in the desired pattern of the rear contact, the solar cell blank having an AR coating (preferably silicon nitride) on its front surface. Then, the solar cell blank is fired in a nitrogen atmosphere at a temperature and for a time adequate to produce an aluminum contact alloyed to the underlying silicon substrate in the manner described above. Thereafter, a silver metal/glass frit paste is coated onto the AR layer so as to define a suitable grid contact pattern, as discussed above, following which a second firing operation is conducted in air to form a silver grid contact bonded to the front surface of the solar cell blank.
When the double fire process involves forming silver soldering pads in opening in the aluminum back contact, the blocks or segments of silver metal paste used to form the pads are fired at the same time as the silver metal paste used to form the front grid electrode. Typically, the silver metal paste used to form the soldering pads contain a glass frit, as does the silver metal paste used to form the front contact that is fired through the AR coating. See International Application No. PCT/US91/06445, filed 6 Sep. 1991 for "Electrical Contacts And Method Of Manufacturing Same", based on U.S. application Ser. No. 586,894, filed 24 Sep. 1990, by David A. St. Angelo et al. (U.S. Pat. No. 5,118,362, issued 2 Jun. 1992), which is incorporated herein by reference thereto.
The so-called "double-fire" process is costly because of the steps and equipment involved. Hence, efforts also have been made to develop a successful so-called "single-fire" process in which the aluminum back contact and a grid-like silver front contact are fired simultaneously.
A primary concern of solar cell manufacturers is the need to decrease contact corrosion and increase reliability and useful life of solar cells and solar cell modules and panels. Although solar cell modules, i.e., modules comprising a plurality of solar cells connected in a suitable series and/or parallel circuit matrix, are made so that the solar cells are sealed between substantially rigid front and back support sheets, with at least the front sheet being transparent, the solar cells are subject to deterioration because of some leakage of outside atmosphere through the protective module encapsulation. Such leakage tends to result in cell deterioration, in part by oxidation and corrosion of the aluminum contacts. Oxidation of the aluminum back contact reduces cell efficiency and also shortens the useful life expectancy of the cells and modules.
Moreover, problems have been encountered in simultaneously firing the paste used to form the silver front contacts and aluminum rear contacts in the same atmosphere. The silver paste must be fired in air. Unfortunately, aluminum oxidation is accelerated when the aluminum-containing coating is fired in an oxygen-containing atmosphere, resulting in a porous aluminum contact on the rear side of the substrate. This porous aluminum metallization tends to degrade rapidly during conventional accelerated testing. Furthermore, there is a strong tendency for the aluminum to form what have been variously referred to as "balls" or "bumps" when fired in air. These anomalies in the rear contact tend to result in an increase in cell breakage in the course of interconnecting and encapsulating a plurality of cells together in a module.
Further, with respect to the single fire process, where silver soldering pads are provided in apertures in the aluminum contact with the silver pads overlapping edge portions of the aluminum layer, it has been found that the silver overlapping the aluminum tends to flake off after firing. Obviously, this poor mechanical adherence between the silver and the aluminum is undesirable.
The prior art also tends to suggest that increasing the thickness of the aluminum contact on the rear side of the solar cell may result in an improvement in overall cell efficiency. The reason for this is not fully understood, but it is believed that since a thicker aluminum contact can be achieved by increasing the amount of aluminum paste applied to the substrate, firing of the thicker paste layer will result in formation of a thicker aluminum metal contact, and also in formation of a thicker aluminum-rich (P.sup.+) region or zone between the aluminum metal contact and the underlying substrate. The latter result provides an improved back surface field, resulting in improved cell efficiency. In this context, what is meant by a "thick" aluminum contact is one with a thickness on the order of 25 microns, in contrast to prior "thin" aluminum contacts which are characterized by a thickness not exceeding about 8 microns. By way of example, a "thick" aluminum contact may be obtained by coating an aluminum paste so as to provide 4.6-8.0 mg of aluminum per cm.sup.2 of coated substrate surface, while a "thin" aluminum contact is produced if the paste is applied in a thickness providing aluminum in an amount equal to between 0.8-2.3 mg/cm.sup.2 of coated substrate surface.
However, it has been determined that providing thick aluminum contacts on thin polycrystalline solar cell blanks, e.g., blanks comprising substrates made by the EFG crystal growth process, is not feasible when using the double fire process, because such thick aluminum contacts have been found to so warp the underlying semi-conductor substrates as a consequence of the double firing as to cause breakage of the cell.